Compared with a liquid crystal display panel in the conventional technology, an OLED (Organic Light Emitting Diode) display panel has characteristics such as a faster response speed, better color purity and brightness, higher contrast, a wider visual angle and so on. Thus, the display technology developers are paying increasingly more attention to OLED display panels.
A pixel unit in the OLED display panel mainly includes an OLED and a pixel unit driving circuit for driving the OLED. FIG. 1 is a schematic diagram of a 2T1C pixel unit driving circuit in the prior art. As shown in FIG. 1, the 2T1C pixel unit driving circuit includes a switching transistor T1, a driving transistor DTFT and a storage capacitor Cst. The switching transistor T1 is controlled by a first scan signal Sn output from a scan line, so as to control the writing of a data signal Data of a data line. The driving transistor DTFT is used for outputting a driving current to the OLED to make the OLED emit light. The storage capacitor Cst is used to provide a maintaining voltage to a gate electrode of the driving transistor DTFT.
The organic light emitting diode OLED can emit light while it is driven by the driving current produced by the driving transistor DTFT operating in a turn-on state, wherein the driving current IOLED may be expressed as:
      I    OLED    =            1      2        ⁢                  μ        n            ·              C        OX            ·              W        L            ·                        (                      Vdata            -            ELVDD            -            Vth                    )                2            
where, μn·Cox·W/L is a constant related to process and driving design, for example, μn is carrier mobility, Cox is gate oxide layer capacitance, W/L is a width to length ratio of the transistor; Vdata is a voltage of the data signal Data, ELVDD is a driving voltage of the driving transistor DTFT and is shared by all the pixel units, and Vth is a threshold voltage of the driving transistor DTFT.
However, due to technical limitations, uniformity of the threshold voltages Vths are usually poor, furthermore, drifting of the threshold voltage Vth will occur in use. From the above equation, it can be known that, if different pixel units have different Vths, differences exists in the driving currents, causing nonuniform of display brightness (for example, which may be up to 50% or more); and if the threshold voltage Vth of the driving transistor drifts over time, it may result in different currents over time, influencing display effect.